Rubber articles are typically reinforced with glass, steel, or polymeric material to provide increased strength, stability and fatigue resistance. For instance, high strength polymeric cords and polymeric fabrics are generally incorporated into tires to improve the physical and performance characteristics thereof. It is also very common to use polymeric cords and fabric to reinforce other rubber articles such as hoses, conveyor belts, power transmission belts and rubber mountings.
Even though alternative materials, such as metals, can be prepared and used to reinforce rubber articles, polymeric reinforcements are used in many instances to reduce the weight of the article, for ease of manufacture, and for improved performance characteristics. Unfortunately, polymeric fiber-to-rubber adhesion tends to be poor in certain specialized systems and may be degraded during use, causing reduced performance and a shortened product life. This is particularly true in cases where polymeric reinforcements are made utilizing materials such as polyaramide fibers which exhibit particularly poor adhesion to rubber compositions.
Various methods have been utilized in the past to promote adhesion and to reduce degradation during the life of the rubber article. One such method which has been widely used commercially throughout the rubber industry involves the application of various resorcinol-formaldehyde-latex (RFL) coatings to the surface of the polymeric reinforcement. Treating the polymeric reinforcement in preparation for RFL dipping treatment, and the RFL treatment itself, involves a complicated sequence of time consuming and consequently expensive steps. U.S. Pat. No. 5,053,246 to Shuttleworth discloses a technique for treating the surfaces of rubber-reinforcing materials with a gas plasma of oxygen and carbon tetrafluoride to activate its surface making it more receptive to adhesive. The method disclosed by Shuttleworth comprises the sequential steps of (1) treating the surface of a polymer in a plasma chamber containing a mixture of oxygen (O.sub.2) and tetrafluoromethane (CF.sub.4) at a pressure of about 0.001 to 1 Torr. in a plasma generated by electrical energy of about 50 Hz to 2.45 GHz; and (2) further treating the polymer with a plasma of an incorporating gas, such as an air/water mixture, generated by electrical energy. In a preferred embodiment, the polymer is in the form of a filament, tow, cord, woven fabric or fiber. The plasma treatment removes hydrogen bonded species and other weakly bonded species, such as sulfates, carbonates, low molecular weight polymer, and processing aids, etc. from chemically reactive sites on the polymer surface, making those sites available for chemical bonding. The chemically reactive sites may then be exposed to other species, such as an RFL adhesive.
Elastomers can be crosslinked by a number of techniques as described by A. Coran in "Science and Technology of Rubber" Chapter 7. Most elastomers are cured using sulfur or peroxide vulcanization although curatives such as phenolic resins, quinone derivatives, maleimide derivatives and metal oxides can also be employed in the crosslinking reaction.
A novel crosslinking reaction was reported in Kautschuk Gummi Kunststoffe 8/83 which discloses a cure system consisting of the adduct of a diisocyanate with nitrosophenol. In this cure system, the nitrosophenol is released at cure temperature and reacts with the elastomer. Crosslinking occurs when the diisocyanate reacts with functionality on the polymer bond nitrosophenol. This blocked isocyanate curative is incorporated in a step separate from the polymerization.
Polymer bound isocyanates have been used as part of a curing package of acrylate polymers. European Patent Application Publication No. 130,322 discloses that acrylate polymers containing m-isopropenyl .alpha., .alpha.-dimethyl benzylisocyanate (TMI) can be cured and that the polymers can be used in coating applications. U.S. Pat. No. 4,694,057 discloses that elastomers containing small amounts of 1-(1-isocyanato-1-methylethyl) -3-(l-methylethenyl)-benzene (TMI) can be crosslinked by utilizing difunctional or polyfunctional active hydrogen compounds. For instance, U.S. Pat. No. 4,694,057 discloses that elastomers which are difficult to cure with sulfur or sulfur containing compounds, such as polychloroprene, can be cured utilizing such a technique.
U.S. Pat. No. 4,983,684 and U.S. Pat. No. 5,043,455 disclose a technique for curing rubber compositions which does not rely upon sulfur or sulfur containing compounds. The cure systems disclosed by these patents rely upon the reaction between a blocked isocyanate group and an active Zerewitinoff hydrogen atom. The utilization of this process accordingly results in a urethane or urea type cure. One benefit realized by utilizing this process is that premature crosslinking of the elastomer is generally not a significant problem if the appropriate blocking group is used.
U.S. Pat. No. 4,983,684 and U.S. Pat. 5,043,455 more specifically disclose a rubber composition which can be crosslinked by heating which is comprised of (1) at least one rubber having pendant blocked isocyanate groups bound thereto; and (2) at least one compound which contains at least 2 Zerewitinoff active hydrogens.